The present invention relates generally to a power amplification circuit and method for supplying power at a plurality of desired power output levels, and more particularly to a power amplification circuit, which adjusts the load coupled to the output of the power amplifier, responsive to the desired power output levels, to enable further reductions in the amplifier bias signal without exceeding predefined distortion limits.
Many wireless communication protocols provide for transmitters, operating within a communication network, which are capable of transmitting at varying levels of output power. One reason for having varying levels of output power is to accommodate mobile transmitters, which may be located at a varying distance from a base station. In some instances, the wireless communication protocol requires that the signal being received by the base station is received at a relatively constant or fixed power level. Examples of two such protocols include Code Division Multiple Access (CDMA) or Wideband Code Division Multiple Access (WCDMA). To accommodate this requirement, a mobile transmitter will transmit at one of several power output levels, dependent upon the level at which the signal is being received.
Other examples, where the transmitted output power can be varied, include the Global System for Mobile communications (GSM), which provides for a range of output power control of mobile transmitters between 20 dB and 30 dB, which is controllable in steps of 2 dB, and earlier analog cellular standards, which call for seven 4 dB steps in power output of the radio transmitter.
Previously, power amplifiers that were capable of delivering a range of power outputs were often designed to operate most efficiently at the highest power level. This is because relatively larger amounts of power are consumed, when the power amplifiers are operating at the highest power levels, than when power amplifiers are operating at lower power levels. Consequently, this has generally resulted in power amplifiers, which sacrificed power efficiencies at lower power levels. However, the mobile station is generally only required to transmit at its maximum power level when the path losses are the greatest. Correspondingly, the mobile station will typically transmit at lower power levels for a larger percentage of the time, that the mobile station is transmitting.
One previous technique, which has been used to enhance operating efficiencies at lower power output values, has included reducing the bias signal supplied to the amplifier. However, there is a limit to the amount that the bias signal can be reduced. Reducing the bias too much will lead to distortion, and increase the likelihood that adjacent channel power requirements will not be met.
A further technique, which has been used to enhance operating efficiencies at lower power output values, is to adjust the load impedance coupled to the output of the power amplifier. However, as the power output value moves further away from the original maximum required, a greater impedance change becomes necessary to maintain performance.
At least one prior reference has attempted to accommodate changes in the transmitter operating environment by making adjustments in the power amplifier circuit, namely, Klomsdorf et al., U.S. Pat. No. 6,281,748, entitled xe2x80x9cMethod And Apparatus for Modulation Dependent Signal Amplificationxe2x80x9d, which is assigned to Motorola, the disclosure of which is incorporated herein by reference. Klomsdorf et al. discloses a multi-mode communications transmitter capable of transmitting a signal having different modulation formats, for example quaternary phase shift keying (QPSK), binary phase shift keying (BPSK), and/or quadrature amplitude modulation (QAM). The transmitter comprises an amplifier having a modulation format switch with different impedance values for the different communication modulation formats.
Klomsdorf et al. also discloses reducing power consumption in the power amplifier by adjusting a bias applied thereto. For further example, see U.S. Pat. No. 5,625,322, entitled xe2x80x9cAmplifier Bias Control Devicexe2x80x9d, among other patents.
However, the present inventors have recognized, that at low power output levels, by reducing the impedance coupled to the output of the power amplifier, the bias signal can be reduced even further, while the amount of noise or power transmitted in an adjacent channel remains below the adjacent channel power requirements. As a result greater efficiencies at lower power output levels can be realized without sacrificing power efficiencies at higher power output levels.
The present invention provides a method in a power amplifier of supplying power to a load at a plurality of different power levels. The method includes changing between high and low power outputs of the power amplifier when amplifying a signal having a common modulation format. A different bias is applied to the power amplifier at the low power output than the bias applied to the power amplifier at the high power output. Additionally, the output of the power amplifier is loaded with a different impedance at the low power output than the impedance loaded at the output of the power amplifier output at the high power output.
In at least one embodiment, the power amplifier changes between high and low power outputs at a different threshold level dependant upon whether the power amplifier is transitioning from the high power output to the low power output, or whether the power amplifier is transitioning from the low power output to the high power output.
In a still further embodiment, a method of supplying power to a load via a power amplification circuit over an extended range includes defining a set of a plurality of desired power output levels, where each power output level is a subset of the extended range. A bias signal supply level and an output impedance is associated with each desired power output levels. A desired power output level is then selected. The bias signal supply level is then adjusted, and an output impedance is then selected, corresponding to the selected desired power output level. A signal is then received by the power amplification circuit and amplified.
The present invention further provides a power amplification circuit, which includes a power amplifier having an output with high and low output power levels within an operating range of the power amplification circuit. The power amplification circuit further includes a variable bias circuit coupled to the power amplifier, where the variable bias circuit has at least a first and second power amplifier bias configurations, and a variable impedance circuit coupled to the output of the power amplifier, where the variable impedance circuit has at least a first and second power amplifier impedance configurations. The power amplifier is configured in a reduced bias configuration at the low output power level relative to the bias configuration of the power amplifier at the high output power level, and the power amplifier is loaded with an increased impedance at the low output power level relative to the loading of the power amplifier at the high output power level.
In a further embodiment, the power amplification circuit includes a control circuit including a bias adjust circuit, and an impedance select circuit, for receiving a signal, which selects one of the desired output power levels, and for producing a bias adjust control signal and an impedance select control signal, which are respectively received by the variable bias circuit and the variable impedance circuit.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.